The present invention relates generally to apparatus for processing an input signal and more particularly to apparatus for compressing the input signal for storage or transmission purposes.
Apparatus for compressing a signal, known as data compression devices, are used to reduce the data storage or the transmission bandwidth requirements of a system. Essentially, the data compression device takes advantage of redundancies in an original data input signal by storing or transmitting only certain samples, i.e., non-redundant information, of the signal. The non-redundant information is the information that is required for reconstructing the original signal without any substantial loss of information.
Prior data compression devices can be classified into two types, one using an amplitude threshold technique and the other using a slope threshold technique. In the amplitude threshold technique, the data compression device analyzes the amplitude of the input signal to determine the sampling rate for this signal. If the amplitude is above the threshold, the data compression device decides that this is non-redundant information and a sample is stored or transmitted. If the amplitude is below the threshold, the signal is considered to be redundant and no sample is stored or transmitted.
Similarly, data compression devices based on the slope threshold technique analyze the input signal for changes in amplitude to determine sampling rate. If the rate of change is above a threshold, the data compression device decides that the signal is non-redundant and a sample is stored or transmitted. If the rate of change is below the threshold, then the signal is considered to be redundant and no sample is stored or transmitted.
Both the prior amplitude and slope threshold data compression devices perform a relatively simple analysis of the content of the input signal for data compression purposes. That is, these devices look for only the amplitude or changes in amplitude of the input signal. This analysis limits the amount of data compression that can be achieved for a given input signal, thereby undesirably increasing the data storage or transmission bandwidth requirements.
Furthermore, in prior data compression devices, typically the circuits which determine the sampling rate of the signal constitute analog components. One problem is that voltage drifts or temperature drifts of the analog components can affect the sampling rate. For example, if there is a voltage drift, a differentiating circuit used in a slope threshold technique may not detect small changes in amplitude of the input signal. Consequently, this portion of the input signal may be non-redundant information, yet the signal may not be sampled and this non-redundant information will not be stored or transmitted for reconstruction purposes.